Bioactive Natural Products from Chinese Marine Flora and Fauna

Review

Bioactive natural products from Chinese marine flora and fauna

Zhen-fang ZHOU, Yue-wei GUO*

State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China

In recent decades, the pharmaceutical application potential of marine natural products has attracted much interest from both natural product chemists and pharmacologists. Our group has long been engaged in the search for bioactive natural products from Chinese marine flora (such as mangroves and algae) and fauna (including sponges, soft corals, and mollusks), resulting in the isolation and characterization of numerous novel secondary metabolites spanning a wide range of structural classes and various biosynthetic origins. Of particular interest is the fact that many of these compounds show promising biological activities, including cytotoxic, antibacterial, and enzyme inhibitory effects. By describing representative studies, this review presents a comprehensive summary regarding the achievements and progress made by our group in the past decade. Several interesting examples are discussed in detail.

Keywords: marine natural products; biological activity; mangrove; algae; soft coral; sponges; mollusks

Acta Pharmacologica Sinica (2012) 33: 1159–1169; doi: 10.1038/aps.2012.110; published online 3 Sep 2012

Introduction ment of Chinese marine natural products. This review sum- The unique ocean habitat has caused marine organisms to marizes the progress and achievements made by our group evolve distinctive metabolic pathways, producing remark- in the study of Chinese marine flora and fauna in the past able secondary metabolites that differ from those of terrestrial decade, and several interesting examples are discussed in plants. Because the compounds isolated from marine organ- detail. isms are structurally and biologically intriguing, research on marine natural products has been attracting the attention of Chemical studies on Chinese marine plants chemists as a challenging research topic. To date, more than Algae 30 000 compounds bearing unusual structures and exhibiting Microalgae play important roles in marine biological systems. various bioactivities have been isolated from marine plants Through their photosynthetic ability, algae are the major pro- and invertebrates. ducers of biomass and organic compounds in the ocean. More In recent years, the interest in natural products from Chi- importantly, many algal metabolites present unique structures nese marine organisms has increased dramatically, mak- and are formed via biosynthetic routes that are quite different ing the annual average number of natural products isolated from those known to produce terrestrial metabolites. from marine organisms increase rapidly. As one of the most Marine dinoflagellates are flagellated organisms with both active research groups in this area, we have been devoted to photosynthetic and heterotrophic attributes. The lifestyle of the investigation of Chinese marine plants and invertebrates these organisms is diverse, and many of them participate in since 2000. In the course of our studies, a series of structurally symbiotic relationships. For some time, the study of dinofla- diverse and biologically interesting compounds belonging to gellates has focused on their toxin production and pigment various structural classes, such as macrocyclic polydisulfides, composition, and only a small number of these organisms have spirodioxynaphthalene, and alkaloids, have been isolated and been found to produce other secondary metabolites. As part characterized. Through these productive studies, we have of our ongoing research on the biologically active substances made important contributions to the research and develop- of Chinese marine organisms, three new polyhydroxylpolyene compounds, lingshuiols A and B (1 and 2) and lingshuiol (3),

have been isolated from the Chinese marine dinoflagellate * To whom correspondence should be addressed. E-mail [email protected] Amphidinium sp collected at Lingshui Bay, Hainan Province, Received 2012-06-17 Accepted 2012-07-12 China[1, 2] (Figure 1). The structures of these compounds are npg www.nature.com/aps Zhou ZF et al 1160

Figure 1. Structures of polyhydroxylpolyene compounds 1–3.

characterized by linear carbon chains, as determined by extensive analyses of 2D NMR spectroscopic data; however, the absolute configurations remain to be determined. In previous studies, polyhydroxylpolyene compounds were found to exhibit antifungal, hemolytic, and antimicrobial activities[3–5]. It was demonstrated that lingshuiol (3) possessed potent in vitro cytotoxic activities against A549 and HL-60 cells, with

IC50 values of 0.21 and 0.23 µmol/L, respectively. Red algae of the genus Laurencia are found throughout the world, mostly in tropical and subtropical regions. Since Irie’s pioneering investigations on Laurencia over the last four decades[6], a number of cuparene- and laurane-derived sesquiterpenoids have been isolated from this genus. The chemi- Figure 2. Structures of sesquiterpenes 4–8. cal investigation of the Chinese red alga Laurencia okamurai Yamada led to the isolation of a new laurane sesquiterpenoid, 3β-hydroxyaplysin (4), and two novel rearranged sesquiter- (laurene type) or 1,2,2 (cuparene type), with the exception of penes, laurokamurenes A and B (5 and 6)[7] (Figure 2). Their laurokamurenes A and B (5 and 6). structures, including their relative stereochemistry, were Two novel aromatic valerenane-type sesquiterpenes, cauler- determined based on a detailed interpretation of 2D NMR pals A and B (7 and 8), were isolated from the Chinese green spectra and comparisons with related known compounds. To alga Caulerpa taxifolia[9] (Figure 2). Compounds 7 and 8 were the best of our knowledge, this is also the first report of the evaluated for their inhibitory activity against human protein isolation of sesquiterpenes containing a 2,2,3-trimethylcyclo- tyrosine phosphatase 1B (PTP1B), a potential drug target pentenyl moiety from a natural source. The sesquiterpenoids for the treatment of type 2 diabetes and obesity, but neither isolated from the red algae of the genus Laurencia in recent compound exhibited a potent PTP1B inhibitory activity. It years can be classified into more than 20 sesquiterpenoid is important to note that caulerpals A and B (7 and 8) are the skeletons[8]. However, in most cases, the three methyl groups only two known compounds with an aromatic valerenane- in the aliphatic portion are located at either positions 1,2,3 type carbon skeleton from a natural source.

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Mangroves Mangroves comprise a large number of various salt-tolerant plants growing in tropical and subtropical intertidal estuarine zones. Mangrove plants are usually categorized into two subgroups, true mangrove and semi-mangrove plants, according to their living environment. True mangrove plants are confined to the typical intertidal mangrove habitats where the seawater salinity is usually 17.0‰–36.4‰. Semi-mangrove plants, however, grow on the landward fringe of the mangrove habitats or in the terrestrial marginal zones that are sub- jected to irregularly high tides. In general, mangrove plants consist of 84 species globally, belonging to 24 genera and 16 families; 14 are semi-mangrove species[10, 11]. Most of our studies involve plants belonging to the Rhizo- phoraceae, Sonneratiaceae, Euphorbiaceae, and Meliaceae families. A series of unusual compounds bearing unprecedented structures were identified in these taxa, some of which showed marked biological activities and are currently in pre- clinical studies. For instance, an unusual novel macrocyclic polydisulfide gymnorrhizol (9) was isolated from the mangrove plant Bru- guiera gymnorrhiza (Rhizophoraceae family). Compound 9 possesses an uncommon carbon skeleton that is character- istic of a 15-membered macrocycle composed of 3 repeated 1,3-dimercaptopropan-2-ol units[12] (Figure 3). The structure of compound 9 was established by extensive spectroscopic studies and confirmed using X-ray crystallography[13]. The first total synthesis of gymnorrhizol (9) was completed by our group with a high overall yield[14] (Scheme 1). Prompted by the unusual structure of gymnorrhizol (9) and its potent inhibitory activity against PTP1B (an IC50 value of 14.9 µmol/L), we investigated the same mangrove species collected from different areas, which led to the isolation of two new macrocyclic polydisulfides, 10 and 11[15]. The structures of 10 and 11 were unequivocally determined through single- crystal X-ray diffraction analysis. A plausible biosynthetic pathway for these cyclic polydisul- Figure 3. Structures of macrocyclic polydisulﬁdes 9–11 and their X-rays. fides was also proposed in which the 1,3-dimercaptopropan- 2-ol unit should be the common building block for the biosynthesis of all of these macrocyclic polydisulfides; the self- sizes gave rise to macrocyclic polydisulfides ranging from cyclization of the 1,3-dimercaptopropan-2-ol unit of different 5-membered to 30-membered polydisulfides or even to larger

Scheme 1. Total synthesis of gymnorrhizol (9).

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polydisulfides. Marine disulfide- and multisulfide-containing metabolites are a special and important class of natural products. Due to the increasing interest in these compounds, the progress in this area was previously summarized by our group[16], encompassing the literature published between 1971 and 2010. To obtain sufficient amounts of the polydisulfides for pharmacological research, the mangrove plant B gymnorrhiza collected from Zhanjiang, Guangdong Province, was investigated. Unexpectedly, instead of the ubiquitous macrocyclic polydisulfides, seven new spirodioxynaphthalene compounds, named palmarumycins BG1–BG7 (12–18) (Figure 4), were found[17]. It is noteworthy that these intriguing compounds were generally considered to be fungal metabolites, even though there were four reports on the isolation of palmarumycins (or preussomerins) from plants[18–21]. The fact that compounds 12–18 were isolated in quite an appreciable quantity indicated that these compounds were produced by B gymnorrhiza; however, we could not rule out the possibility that they were the metabolites of an endophytic or epiphytic fungus. The absolute configurations of metabolites 12–18 were determined using time-dependent density functional theory electronic circular dichroism (TDDFTECD) calculations of the solution conformers. It was found that compound 16 displayed an inhibitory activity against the HL-60 and MCF-7 cell lines. Additionally, we published a report sum- marizing the literature on the isolation, structure elucidation, Figure 5. Structures of compounds 19 and 21–25. biological activities, biosynthesis, and chemical synthesis of spirodioxynaphthalenes over the last 20 years[22]. Although the chemical investigation of the plants belonging to the Sonneratiaceae family was not as fruitful as that of the journal of Natural Product Reports as “hot off the press”[24]. Rhizophoraceae family, a novel α-alkylbutenolide dimer, para- In addition, three known compounds, integracins A and B caseolide A (19), was isolated from the mangrove plant Son- (21 and 22) and 15-dehydroxy-integracin B (24), were isolated neratia paracaseolaris (Sonneratiaceae family); this compound is for the first time from the Chinese mangrove plant Sonneratia characterized by an unusual tetraquinane oxa-cage bislactone hainanensis[25] (Figure 5). Although compounds 21–24 had been skeleton bearing two linear alkyl chains[23] (Figure 5). More- reported previously, their absolute configurations were not over, paracaseolide A (19) exhibited a potent inhibitory activ- resolved until their absolute chemistry was determined using ity against the dual-specificity phosphatase Cdc25B, a key Mosher’s method, the specific rotation analysis of the alcohols

enzyme for cell cycle progression, with an IC50 value of 6.44 (24 and 25) obtained from integracin A (21) in two steps, and µmol/L. A retrosynthetic pathway for paracaseolide A (19) chemical correlation. Integracin A (21) exhibited a cytotoxic was proposed in which 19 should be biosynthesized through a effect against the tumor cell lines HepG2 and NCI-H460, both Diels-Alder [4+2] cycloaddition, starting from two molecules of which were 100% inhibited at 25 mg/mL. of 20 (Scheme 2). Paracaseolide A (19) represents the first The Euphorbiaceae family is composed of various species example of an α-alkylbutenolide dimer from a natural source. that are distributed widely throughout China. Although poi- Upon publication, the article was immediately covered in the sonous, most of the plants in this family are utilized in folk

Figure 4. Structures of spirodioxynaphthalene compounds 12–18.

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Scheme 2. Plausible retrosynthetic pathway for paracaseolide A (19). medicine. Thus, the substances of the mangrove plant Excoe- challenging targets for total synthesis, bioactivity evaluation, caria agallocha L (Euphorbiaceae family for the first time) were and biosynthetic studies. Chemical investigation on the stem investigated, which led to the isolation of four diterpenes with bark of the plant Toona ciliata var pubescens resulted in the an unprecedented skeleton, named excoagallochaols A–D isolation of nine new compounds, including toonapubesin A (26–29)[26] (Figure 6). In addition to these terpenes, a novel (31), toonapubesic acid A (32), and their derivatives[28] (Figure octacyclic alkaloid with an unusual scaffold, suffruticosine 6). The proposed structure of 32 was confirmed by the X-ray (30), was isolated from Securinega suffruticosa (Pall) Rehd in diffraction analysis of its methyl ester, whereas the absolute the same family[27] (Figure 6). The absolute configuration of configuration was determined by a novel solid-state TDDFT suffruticosine (30) was determined on the basis of CD spectral ECD approach on its methyl ester. analysis. More importantly, macrolide (R)-de-O-methyllasiodiplodin (33), isolated from the semi-mangrove Cerbera manghas, was found to be a potent nonsteroidal antagonist of the mineralocorticoid receptor (MR), a drug target for the treatment of hypertension and other cardiovascular diseases, showing an

IC50 value of 8.93 µmol/L. Due to its low natural yield, further study on its structural derivatization/modification and the structure-activity relationship (SAR) was conducted. Conse- quently, a series of analogs were synthesized and evaluated for their MR antagonistic activities[29], and compounds 34–36 were found to exhibit a better selectivity and more potent

activity against MR than compound 33, with IC50 values ranging from 0.58 to 1.11 µmol/L.

Chemical studies on Chinese invertebrates Sponges Sponges are animals of the phylum Porifera, which consists of approximately 5 000–10 000 species that can be classified mainly according to the composition of their skeletons as cal- carea, glass sponges, and demosponges. Their chemical vari- ability results from the individual species, particular metabo- lism and complex environment. In the past half century, a large number of compounds, including terpenoids, alkaloids, and polyacetylenes, were isolated and characterized from various sponges collected from different regions of the world. For instance, five new sesquiterpenes, lingshuiolides A and B (37 and 38), lingshuiperoxide (39), isodysetherin (40), and spirolingshuidolide (41) were isolated from the Hainan sponge Dysidea septosa[30] (Figure 7). The absolute configuration of lingshuiolide B (38) was established by using a modi- Figure 6. Structures of diterpenoids and the alkaloid 26–36. fied Mosher’s method. The chemical study on another Dysidea sp led to the discovery of a new sesquiterpene quinone (42), along with a known related analog dysidine (43)[31] (Figure Plants of the Meliaceae family are a rich source of limonoids, 7). A bioassay showed that dysidine (43) had a potent PTP1B which are a structurally diverse group of natural products inhibitory activity, with an IC50 value of 6.70 µmol/L, whereas with highly complex polycyclic skeletons. These unusual the new compound 42 exhibited a moderate PTP1B inhibitory structural features have attracted the attention of chemists as activity and cytotoxicity, with IC50 values of 39.50 and 19.45

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Figure 7. Structures of compounds 37–47.

µmol/L, respectively[32]. Three unprecedented diastereoisomeric dimers, cis-dimers A (44) and B (45) and trans-dimer C (46), representing bis- bisabolene skeletons, and their potential precursor dehydrotheonelline (47) have been isolated from the South China Sea sponges Axinyssa variabilis and Lipastrotethya ana[33] (Figure 7). The three diastereoisomers (44–46) were possibly biosynthesized through an intermolecular [4+2] Diels-Alder cycloaddition involving two molecules of dehydrotheonelline (47). The trans- and cis-dimers were possibly formed through exo- and endo-Diels-Alder coupling, respectively (Scheme 3). Fur- thermore, a mixture of cis-dimers 44 and 45 and the pure trans- dimer 46 were active at 50 mg/cm2 in a feeding-deterrence test against the gold fish Carassius auratus, suggesting a possible defensive role for these compounds.

Soft corals Soft corals from the South China Sea have been extensively studied by Chinese chemists and have yielded a plethora of steroids and terpenoids. It has been suggested that such secondary metabolites produced by soft corals are most likely Scheme 3. Possible pathways for compounds 44–46. involved in the defensive mechanisms of the animals; indeed, the animals appear to be relatively free from predation. As expected, the chemical study of the soft coral Lobophytum cristatum Tixier-Durivault led to the isolation of two new pre- diterpenoids, lobophytumins E and F (52 and 53)[34] (Figure 8). nylgermacrane-type diterpenoids, lobophytumins A and B (48 Their structures, including the relative configurations, were and 49), two new prenyleudesmane-type diterpenoids, lobo- determined via a detailed analysis of the spectroscopic data phytumins C and D (50 and 51), and two new spatane-type and comparison with related known compounds. In addi-

Figure 8. Structures of diterpenoids 48–53 from L cristatum.

Acta Pharmacologica Sinica www.chinaphar.com npg Zhou ZF et al 1165 tion, the absolute configuration of lobophytumin C (50) was between the A and B rings. tentatively assigned by comparing its specific rotation with The study of Sarcophyton lactum also led to the discov- that of the closely related model compound (–)-β-selinene; in ery of two unprecedented biscembranoids, bislatumlides A contrast, the stereochemistry of C-11 in compounds 48–49 and and B (69 and 70), for which the dienophilic double bond 52–53 remains undefined. This is the first report of spatane- belonged to an α,β-conjugated-γ-lactone, along with a new [34] type diterpenoids from a soft coral source , and it supports cembranolide diterpene, isosarcophytonolide D (71)[38] (Figure Faulkner’s proposal of prenylgermacrene as the precursor of 10). Compounds 69 and 70 differ in the configuration of the many diterpenes, including quite different but biogenetically double bond at C-11: E for 69 and Z for 70. However, it was related ones. In a bioactivity evaluation, lobophytumins C proven that the isomerization of the double bond in 69 readily and D (50 and 51) showed weak in vitro cytotoxicities against occurred in the presence of trace amount of acid. Compounds the tumor cell lines A549 and HCT-116. 69 and 70 could be biosynthesized by the Diels-Alder addi- Casbane diterpenes are characterized by the presence of a tion of two cembranoid units. Compound 71 and the known dimethyl-cyclopropyl moiety fused to the 14-membered ring, compound sarcophytonolide D (72) may comprise half of the structures that are extremely rare in nature and marine organ- biosynthetic precursors, as shown in Scheme 4. isms and previously found only in Sinularia microclavata[35]. From the Hainan soft coral Sinularia depressa, we isolated nine casbane diterpenes (54–62), either cis or trans ring junc- tions, including two pairs of epimers: depressin (54) and 1-epi- depressin (55) and 10-hydroxydepressin (56) and 1-epi-10- hydroxydepressin (57)[36] (Figure 9). Compounds 56 and 57 played a key role in the establishment of the absolute stereochemistry, as the S configuration at C-10 of both compounds could be easily assigned using Mosher’s method. Three fur- Scheme 4. Plausible Diels–Alder reaction leading to compounds 69 and ther casbanes (59–61) displayed identical structures, with the 70. exception of the stereochemistries at the ring junction, which were cis in 59 and trans in both 60 and 61. Compound 58 was similar to 57, except for the oxidization of the hydroxyl; Steroids are a large group of compounds encountered in nevertheless, compound 62 had many similarities with marine organisms and are reported to display various bio- 10-hydroxydepressin (56), with the exception of an additional activities. For instance, a novel steroid with an uncommon oxygen atom. 10-Hydroxydepressin (56) showed a cytotoxic 21-oic acid methyl ester moiety, designated methyl spongoate activity against the tumor cell lines HepG2 and SW-1990, with (73), was isolated from the Sanya soft coral Spongodes sp; IC50 values of 61 and 37 µmol/L, respectively, and an antimi- this steroid exhibited a potent cytotoxicity against BEL-7402 crobial activity against Staphylococcus aureus and Escherichia tumor cells in vitro[39]. Later, its stereoselective synthesis was coli at the concentration of 17 µmol/L. achieved in our laboratory[40]. Owning to its unusual structure The genus Sarcophyton is a productive source for unusual and potent cytotoxicity, a number of analogs with different tetraterpenoids, which are constructed by coupling two cem- C-20 side chains were synthesized and evaluated for their branoid units through the Diels-Alder reaction. Five novel cytotoxic activities, and some of the analogs were found to be biscembranoids, ximaolides A–E (64–68), together with their more potent than compound 73 against the A549, HCT-116, proposed biogenetic precursor, methyl tortuosoate (63), were HepG2, SW-1990, MCF-7, and NCI-H460 tumor cell lines. The [37] isolated from the Hainan soft coral Sarcophyton tortuosum preliminary SAR study suggested that the unsaturated car- (Figure 10). The relative stereochemistries of ximaolides A bonyl moiety, a Michael acceptor in ring A, and the C-20 side and E (64 and 68) were acquired by X-ray diffraction analysis, chain played important roles in the cytotoxic effect of these whereas the stereochemistries of the other compounds were derivatives[41] (Figure 10). suggested by both biosynthetic considerations and NOESY experiments. Furthermore, ximaolide D (67) is the first Mollusks example of a biscembranoid possessing a tetrahydrofuran ring Mollusks are a large phylum of invertebrate animals that are

Figure 9. Structures of casbane diterpenes 54–62.

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Figure 10. Structures of biscembranoids 63–73.

highly diverse in size and anatomical structure and also in analysis, whereas the S configuration of the hydroxyl at C-6 their behavior and habitat. Mollusks show such a variety in in tritoniopsin C (76) was assigned by applying Mosher’s body structures that it is difficult to find definite characteris- method. Both tritoniopsins A and B (74 and 75) were discov- tics that apply to all of the classification groups. The two most ered from T elegans and C krempfi, but, surprisingly, the major universal features are a mantle with an important cavity used compound in this mollusk was tritoniopsin A (74). Such an for breathing and excretion and the structure of the nervous uneven metabolite distribution could be explained by the fact system. However, most of the animals in the Opisthobranchia that the mollusk either modified dietary tritoniopsin B (75) or family lose their mantles and require other defensive system selectively accumulated compound 74. The presence of these for protection. Thus, these animals usually produce structur- unique metabolites in both the nudibranch and the soft coral ally diverse substances to form a chemical defensive system, C krempfi also confirmed the trophic relationship between the representing a different source of bioactive compounds from predator and prey. marine organisms. As expected, a series of metabolites with During the investigation of the nudibranch Glossodoris complex structures and intriguing biological activities have rufomarginata and its unidentified prey sponge from the been isolated from mollusks. South China Sea, it was observed that scalaradial (78) and The study on the South China Sea nudibranch Tritoniopsis its deacetyl derivative (79) were the main components of the elegans led to the isolation of four diterpenes, tritoniopsins A-D extract of the dietary sponge, whereas a series of related sca- (74–77), which display an unprecedented pyran ring in the laranes (80–84) (Figure 11), most likely derived from dietary cladiellane framework and represent a novel cladiellane-based scalaradial, were isolated from the mollusk, thus proving diterpene family[42] (Figure 11). These compounds were also the ability of Glossodoris rufomarginata to modify dietary mol- found in the nudibranch’s prey, Cladiella krempfi. The struc- ecules[43]. The finding of these molecules (80, 81, and 84) with ture of tritoniopsin A (74) was elucidated by X-ray diffraction a keto functionalization at C-12 is particularly interesting.

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Figure 11. Structures of the compounds 74–92 from mollusks.

This structural feature is quite common in the homoscalarane was present both in the mucus and in the mantle[47]. The struc- series but unusual in the scalaranes, and in particular it has ture of the polypropionate was suggested by an analysis of been encountered only in nudibranch metabolites. In fact, in spectral data. In particular, the relative stereochemistry of the addition to 12-deacetoxy-12-oxo-deoxoscalarin (80), previ- central ring was supported by a series of nuclear overhauser ously isolated from Indian Glossodoris atromarginata[44], only enhancement (NOE) experiments, and the complete structure one 12-keto-scalarane, 12-deacetoxy-12-oxo-18-epi-scalaradial, was confirmed by X-ray diffraction analysis. A recent study from Pacific nudibranch Chromodoris youngbleuthi[45], has been on the second collection of the mollusk led to the isolation previously reported. of onchidiol (88) and 4-epi-onchidiol (89), related alcohols of In our research on the sea hare Aplysia dactylomela, two onchidione (87)[48] (Figure 11). The absolute configurations novel triterpenes, aplysiols A and B (85 and 86), characterized of onchidiol (88) and 4-epi-onchidiol (89) were established by as squalene-derived polyethers, were isolated[46] (Figure 11). TDDFTECD. Moreover, the absolute chemistry of onchidione The absolute stereochemistry of aplysiol A (85) was deter- (87) was corroborated by X-ray diffraction analysis, with the mined using Mosher’s method and biogenetic considerations, final refinement using Cu-Kα data. whereas the configuration of aplysiol B (86) was established In the final section of this review, we describe the nitrogen- by an integrated NMR-QM (quantum mechanical) approach containing compounds of marine mollusks. The first example 13 2,3 on the basis of C NMR chemical shifts and JC–H coupling- consists of phidianidines A and B (90 and 91) obtained from constant DFT (density functional theory) calculations. Phidiana militaris, belonging to the Glaucidae family[49] (Figure Marine pulmonata of the Onchidiidae family are shell-less 11). The two novel alkaloids were characterized with an mollusks living on sheltered intertidal shores. Previous chem- unprecedented carbon skeleton featuring the presence of a ical studies on distinct species in this family mostly focused on 1,2,4-oxadiazole ring linking the indole system through a polypropionates. In our study, onchidione (87), as the main methylene bridge and displaying an aminoalkylguanidine metabolite, was characterized from the first collection of the group at C-3’. The 1,2,4-oxadiazole ring is extremely rare in Chinese marine pulmonate Onchidium sp (Figure 11), which nature. The phidianidines were tested in a panel of tumor

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cell lines (C6 rat glioma cells, HeLa human epithelial cervical to thank Prof T Kurtán (University of Debrecen, Hungary) cancer cells and CaCo-2 human epithelial colorectal adenocar- for his contribution to the establishment of absolute stereo- cinoma cells) and non-tumor cell lines (H9c2 rat embryonic chemistry by CD methodology. The reported research was cardiac myoblasts and 3T3-L1 murine embryonic fibroblasts) financially supported by the National Marine ‘863’ Project (No and showed a strong and selective activity against C6 and 3T3- 2011AA09070102), the National Natural Science Foundation

L1 cells by IC50 values within the nanomolar range, as shown of China (No 21072204, 21021063, 40976048, 31070310, and in Table 1. 81072572), the SKLDR/SIMM Projects (No SIMM 1203ZZ-03 and SIMM 1105KF-04), and Syngenta-SIMM-PhD Student- Table 1. Cytotoxicity profle of compounds 90 and 91 against tumor and ship Project, and partially funded by the National S&T Major a non-tumor cell lines, IC50 (µmol/L) . Project (2011ZX09307-002-03) and the EU 7th Framework Programme-IRSES Project (2010-2014). Cell line Phidianidine A (90) Phidianidine B (91) References C6 0.642±0.2 0.98±0.3 1 Huang XC, Zhao D, Guo YW, Wu HM, Trivellone E, Cimino G. Ling HeLa 1.52±0.3 0.417±0.4 shuiols A and B, two new polyhydroxy compounds from the Chinese CaCo-2 35.5±4 100.2±8.5 marine dinoflagellate Amphidinium sp. Tetrahedron Lett 2004; 45: 3T3-L1 0.14±0.2 0.786±0.3 5501–4. H9c2 2.26±0.6 5.42±0.8 2 Huang XC, Zhao D, Guo YW, Wu HM, Lin LP, Wang ZH, et al. Lingshuiol, a novel polyhydroxyl compound with strongly cytotoxic a IC50 values are expressed as mean±SEM (n=24) of the three independent activity from the marine dinoflagellate Amphidinium sp. Bioorg Med experiments. Bold values show IC50 of less than 1 µmol/L. Chem Lett 2004; 14: 3117–20. 3 Doi Y, Ishibashi M, Nakamichi H, Kosaka T, Ishikawa T, Kobayashi J. Luteophanol A, a new polyhydroxyl compound from symbiotic marine The second example is an unusual molecule, actisonitrile dinoflagellate Amphidinium sp. J Org Chem 1997; 62: 3820–3. (92), found in the mantle of Actinocyclus papillatus[50] (Figure 4 Paul GK, Matsumori N, Murata M, Tachibana K. Isolation and 11), belonging to the Actinocyclidae family that is another chemical structure of amphidinol 2, a potent hemolytic compound family of opisthobranch, which was never studied thus far. from marine dinoflagellate Amphidinium klebsii. 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Conclusions 8 Mao SC, Guo YW. Sesquiterpenes from algae of the Genus Larencia: This review summarizes representative substances from Chi- chemistry and biological activities. Chin Tradit Herb Drugs 2004; 35: nese marine plants and invertebrates found by our group, 8–18. indicating that marine organisms are an inexhaustible source 9 Mao SC, Guo YW, Shen X. Two novel aromatic valerenane-type sesqui terpenes from the Chinese green alga Caulerpa taxifolia. Bioorg Med of new molecules that often display unique structures and Chem Lett 2006; 16: 2947–50. sometimes have very interesting pharmacological proper- 10 Wang BS, Liang SC, Zhang WY, Zan QJ. Mangrove Flora of the world. ties, such as antifungal, antibacterial, enzyme-inhibitory, and Acta Bot Sin 2003; 45: 644–53. other activities. Some of the compounds isolated from marine 11 Li MY, Xiao Q, Pan JY, Wu J. Natural products from semi-mangrove organisms even show promising activities within the nanomo- flora: source, chemistry and bioactivities. Nat Prod Rep 2009; 26: lar range and can be developed as drug candidates or at least 281–98. lead compounds in the near future. Although our research on 12 Sun YQ, Guo YW. Gymnorrhizol, an unusual macrocyclic polydisulfde marine natural products is limited, it has established the foun- from the Chinese mangrove Bruguiera gymnorrhiza. Tetrahedron Lett dation for further studies. 2004; 45: 5533–5. Because our understanding of the biosynthetic origin and 13 Sun YQ, Guo YW. Crystal structure of 1,2,6,7,11,12-hexathia-cyclo the real ecological role of these bioactive compounds remains pentadecane-4,9,14-triol, C9H18O3S6. Z Kristallogr NCS 2004; 219: 121–3. incomplete, further research to clarify these important issues 14 Gong JX, Shen X, Yao LG, Jiang H, Krohn K, Guo YW. Total synthesis is necessary. of gymnorrhizol, an unprecedented 15-membered macrocyclic polydi sulfde from the Chinese mangrove Bruguiera gymnorrhiza. Org Lett Acknowledgements 2007; 9: 1715–6. The authors wish to acknowledge all of their colleagues 15 Huang XY, Wang Q, Liu HL, Zhang Y, Xin GR, Shen X, et al. Diastereo who have contributed to the studies presented herein and isomeric macrocyclic polydisulfides from the mangrove Bruguiera

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